It is important to minimize the size of the elements in a MOS integrated circuit device to increase the integration density of the device. However, the electrical characteristics of the integrated circuit device become remarkably lower as the deveice size becomes smaller. One of the problems cause by the miniaturization of the device size is reduction of a break down voltage between a source and a drain and another problem is reduction of the threshold voltage. These problems are referred to as short-channel effects because these undesirable phenomena become more remarkable as the device size becomes smaller. The short-channel effect is disclosed in Device Characteristics of Short Channel and Narrow Width MOSFET's by Paul P. Wang, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-25, NO. 7, on Pages 779-786, 1978.
According to the literature, in order to reduce the short-channel effect, there is proposed to form an ion implantation layer around the channel region of MOSFET's.
It is well known that a punch through between a source and drain regions in IGFET does not occur on the surface of a substrate in which IGFET's are fabricated but occurs in a relatively deep region in the substrate.
The said deep ion implantation layer serves effectively to increase the break-down voltage. This is apparent from the following experiment.
FIG. 1 shows a schematic cross sectional view of an IGFET used in the experiment.
In FIG. 1, the IGFET comprises a substrate 1 made of P-type semiconductor, a source region 2, a drain region 3 separated from the source region 2 with a given distance, the respective source 2 and drain 3 are formed near the surface of the substrate 1. A gate dioxide layer 4 is disposed on the surface of the substrate 1 opposing to the channel region defined between the source 2 and drain 3; on the gate dioxide layer 4 a gate electrode 5 made of polycrystalline silicon material is formed.
The channel length L shown in FIG. 1 is a distance between the both opposed ends of the source 2 and drain 3 on the surface of substrate 1. FIG. 2 is a characteristic curves showing the relationships between the channel length L on the X axis and the break-down voltage of the drain on the Y axis wherein value of the deep ion implantation is taken as parameters.
It is apparent from FIG. 2, that in the IGFET's without ion implantation layer the break-down voltage of the drain is significantly reduced as the channel length becomes shorter as shown by the dot marks. On the other hand, as shown by the circle or triangular marks, in the ion implanted IGFET's the break-down voltage can be maintained high when the channel length is to 2 to 3 .mu.m, although there are variations of the break-down voltage depending on the amount of the ion implantation.
From the foregoing, it is understood that in the short channel IGFET's, the deep ion implantation layer serves to improve the break down voltage.
Observation on the threshold voltage of the short channel IGFET's is described hereinafter. It is apparently shown in FIG. 3 by the dot marks that in the conventional IGFET's without deep ion implantation layer, the threshold voltage becomes remarkably low as the channel length becomes shorter. On the other hand, as shown by the circular marks and the triangular marks, in the IGFET with the deep ion implantation layer the channel length dependence of the threshold voltage is improved.
However, in the IGFET's with the deep ion implantation layer, the short-channel effect appears when the channel length is further shortened. The short channel effect of the threshold voltage under the ion implanted IGFET's has only been described in the transaction of International Electron Device Meeting, 1978 on pages 468 to 471 by Yeh et al.
The technique of the deep ion implantation has been adopted to improve the break-down voltage in the short channel IGFET's, however, so far as the threshold voltage is concerned, the effect of the deep ion implantation technique has been merely incidentally discussed.
As understood from the foregoing the conventional IGFET's having the ion implantation layer around the channel region show such a drawback that the short channel effect, particularly the reduction of the threshold voltage due to the reduction of the channel length (referred to as a channel length dependence of the threshold voltage) can not be effectively improved thereby resulting in preventing the integrated circuit device size from minitiralizing to the desired value.